If you want to get back, at a min., Most of your 9mm brass, do you mark it with something? If no, start doing that AND let your squad mates know "this brass marked like this is mine, if you are picking up brass I'd really appreciate if I could get it back" or something like that. At local matches you probably already know a large% of the shooters and they know you and most people are generally nice enough to help you out. Goes the opposite way too, if someone else marks their brass, return it to them or ask who's it is.
If you are not marking your brass then it's fairgame since so many are shooting 9mm, anyone reloading it AND willing to pick it up is going to grab as much as they can.
Hope this helps